The present disclosure relates generally to automobile steering systems and, more particularly, to a method of implementing active deadtime control of switching inverters associated with electric machines, such as electric power steering (EPS) motors, for improved torque ripple performance.
Electric power steering (EPS) has been recently implemented in motor vehicles to improve fuel economy and has even started to replace hydraulic power steering in certain vehicles. One way to accomplish improved fuel economy is through the reduction or elimination of losses inherent in traditional steering systems. To this end, electric power steering requires power only on demand. Commonly, in such EPS systems, an electronic controller is also configured to require significantly less power under a small or no steering input condition. This dramatic decrease from conventional steering assist is the basis of the power and fuel savings.
A polyphase permanent magnet (PM) brushless motor is typically used in EPS systems as the actuator for providing a mechanical assist to the steering mechanism of a vehicle. Such a motor is generally excited with a sinusoidal field to provide lower torque ripple, noise, and vibration as compared to those motors excited with a trapezoidal field. Theoretically, if a motor controller produces polyphase sinusoidal currents with the same frequency and phase as that of the sinusoidal back electromotive force (EMF), the torque output of the motor will be a constant, and zero torque ripple will be achieved. However, due to practical limitations of motor design and controller implementation, there are always deviations from pure sinusoidal back EMF and current waveforms. These deviations usually result in parasitic torque ripple components at certain frequencies and magnitudes. Various methods of torque control can influence the magnitude and characteristics of this torque ripple.
In EPS drive systems based on a voltage mode controlled sinusoidal PM drive, a full bridge power inverter is employed to apply a pulse width modulated (PWM) voltage across the motor phases. Unfortunately, these inverters (particularly those used for sinusoidal brushless motors) suffer from several linearity issues. One such linearity issue results from the application of deadtime in the switching of the inverter's power transistors. The term “deadtime” refers to the amount of time during which there is a delay in the turning on/off of a particular transistor pair. The intent of deadtime is to avoid potential short circuits across the power supply of the EPS motor resulting from propagation delays and errors in the timing of the turn on (conduction) and turn off of the transistors. Accordingly, it is desirable to reduce the effects of torque ripple and non-linearity associated with the application of deadtime to the inverter switching circuitry, thereby enhancing the overall performance of the EPS system.